In multi-color printing technology it has been generally known to regularly calibrate the printing units for the individual color separations, so that printing of the individual color separations in good register is ensured. For example, various calibrating methods for a multi-color printing machine have been known from the Applicant's document DE 101 39 310 A. Such a basic calibration, which is used to calibrate the length tolerances of the printing machine in the region of the printing units is a requirement to allow the machine to subsequently print in register. During this basic calibration, the “exposure timing”, i.e., the time-related enabling of the writing devices of the printing units of the printing machine is set. The objective of the entire calibration is to measure registration errors caused by length tolerances of the machine (distances of the printing units), tolerances of photoconductor rollers (imaging drums) and of rubber-sheet cylinders (intermediate drums) and by the transport belt itself, and to store said errors for future printing operations and pilot-actuation.
For such a known calibration method, as a rule, a plurality of register marks composed of individual, spaced apart lines of the individual colors are printed. The distances between the individual lines are determined by a register sensor that is arranged downstream of the printing units. To do so, the register sensor measures light/dark and dark/light transitions between the individual register lines and the printing material underneath, said printing material usually being a transparent transport belt that is disposed to transport the sheets to be printed through the printing units. When detecting the individual register lines of the register marks, the register sensor requires that sufficient light/dark or dark/light contrasts be provided between the printing material (in this case, the transport belt) and the register lines, because the register sensor, as a rule, is adjusted in such a manner that it only detects contrasts above a certain threshold value. Indeed, it would also be possible to lower the threshold value for the contrast, however, this could potentially result in a plurality of erroneous detections that are not caused by register lines.
In multi-color printing machines, the technical problem now arises that, in recent times, special inks are used more and more frequently, such as, for example, a colorless dry toner that is also referred to as clear dry ink (CDI). Those inks may potentially not exhibit a sufficient contrast with respect to the printing material in order to ensure an appropriate detection above the pre-specified threshold value on the register sensor.
In order to solve this technical problem in the past, the register line of the CDI was, for example, printed on a previously printed black background line during the calibration. Now the CDI again exhibited sufficient contrast with respect to this black background line to thus allow a detection above the threshold value of the register sensor. However, in conjunction with this, the problem arose that with large tolerances within the printing machine, it cannot always be ensured that the CDI falls on the printed black background line. Consequently, a proper detection of the CDI register line was not possible, thus leading to an interruption of the entire calibration process. In these cases, customized machine control software versions were issued for such individual machines, which, under normal circumstances, should be strictly avoided.